Liquefied Petroleum Gas (LP-gas or LPG) is a relatively clean-burning hydrocarbon fuel of mixed composition. It is used primarily as a heating fuel in the United States, but also has found widespread utility as a cooking and vehicle fuel and as a refrigerant.
In North America, propane is the major constituent of LP-gas, with minor amounts of propylene, butane, and/or butylene depending on the particular grade. Because these components are gases at ambient temperature and pressure, LP-gas is stored under pressure in steel tanks of varying size depending on their place in the distribution chain.
Retail LP-gas is distributed from bulk plants where it is stored in large spherical or cylindrical tanks, often having capacities from 10,000 up to 30,000 gallons. The bulk plants are supplied by large multi-axel transport trucks carrying 18,000 gallon tanks which have been filled with LP-gas at the refinery or pipeline. From the bulk plant, LP-gas is shipped to retail customers in single-axel delivery truck called bobtails which have varying capacities, but are usually from 1,800 gallons up to 3,500 gallons. The bobtails are used to fill larger stationary tanks called “pigs,” installed on the customer's property.
The Liquefied Petroleum Gas Code, also known as “Pamphlet 58,” issued by the National Fire Protection Association (NFPA), requires that LP-gas tanks be retrofit to add or modify internal valves by Jul. 1, 2011. To accomplish the retrofit, the propane containers will have to be emptied of both liquid and gaseous propane. Liquid propane, which fills about 80-85% of the volume in a full container, must first be drained. Removal of the remaining propane vapor is conventionally accomplished by a controlled burn-off, which can take several days for a large stationary 30,000 gallon tank.
According to the National Propane Gas Association, there are an estimated 7,000 LP-gas transport trucks, 35,500 bobtails, and 17,000 railcars servicing over 14 million residential LP-gas customers and over one million commercial LPG customers. To bring all of these tanks into compliance with NFPA Pamphlet 58, a massive volume of propane must be burned off if the conventional practice is maintained. Controlled burns are not only time-consuming and potentially dangerous, but wasteful of an increasingly expensive commodity.
It is an object of the present invention to provide a method and system for extracting propane-containing vapors from pressurized storage tanks, without the need for a controlled burn-off, whereby the vapors are condensed to provide a propane-containing liquid.